In set theory, one of the subjects we can address is the Cartesian Product of two Sets. In it we talk about ordered pairs, the pair axiom and the Cartesian product. Ordered pair Before we address the subject of the Cartesian product of two sets, we need to talk about ordered pairs. Let $A$ and $B$ be two arbitrary sets. For $a \in A$ and $b \in B,$ and using the specification axiom we can construct the set